With recent increase in volume of data communication, the need for a mobile communication system that has a higher frequency usage efficiency has been increased and various studies on one cell reuse cellular system that uses the same frequency band in all the cells have been proceeded. In E-UTRA (Evolved Universal Terrestrial Radio Access) system which is one of one cell reuse cellular systems and has been advanced to be standardized by mainly 3GPP (3rd Generation Partnership Project), the OFDMA (Orthogonal Frequency Division Multiple Access) scheme and the SC-FDMA (Single Carrier-Frequency Division Multiple Access) scheme have been discussed as the most favorable candidates for the downlink transmission scheme and uplink transmission scheme, respectively.
Of these, the OFDMA scheme is a scheme in which the user makes access in resource block units that are divided in time and frequency, using OFDM signals that are excellent in robustness against multi-path fading. Since this scheme has high PAPR (Peak-to-Average Power Ratio) performance, it is not suitable as the uplink transmission scheme that is severely limited as to transmission power. In contrast to this, since the SC-FDMA scheme can keep the PAPR performance low compared to OFDM and other multi-carrier schemes so as to obtain a wide coverage, this scheme is suitable for uplink transmission (non-patent document 1).
FIG. 10 shows a terminal apparatus configuration when this SC-FDMA scheme is used for uplink transmission. As shown in FIG. 10, in the terminal apparatus using the SC-FDMA scheme, error correction coding of transmitting data is performed first in an encoder 1000, then the data is modulated at a modulator 1001. Next, the modulated transmitting signal is serial-to-parallel converted by a S/P (Serial to Parallel) converter 1002, the converted signals are then transformed into frequency-domain signals by a DFT (Discrete Fourier Transform) unit 1003. The thus transmitting signals transformed into frequency-domain signals are allocated to sub-carriers for transmission use at sub-carrier mapping unit 1004. Allocation at this point is performed based on the mapping information that was transmitted from a base station apparatus, received by a receiving antenna unit 1011, passed through a radio unit 1012 and an A/D (Analog to Digital) converter 1013 and demodulated at a receiver 1014 while zero is inserted to the sub-carriers unused for transmission. At DFT unit 1003, time-frequency transformation of the same size as the number of sub-carriers that constitute one sub-channel defined in the system is performed. All the signals after time-frequency transformation are allocated to given sub-carriers (sub-channels) and transmitted. For example, when the number of sub-carriers that constitute one sub-channel is 12, the size of time-frequency transformation carried out at DFT unit 1003 is also 12, indicating that all the outputs from DFT unit 1003 are input to sub-carrier mapping unit 1004.
As the allocation method at this point, in E-UTRA system, an allocation method called localized allocation that uses contiguous sub-carriers or an allocation method called distributed allocation that uses sub-carriers located a constant distance apart have been discussed. FIG. 11 shows these two allocation examples. FIGS. 11(a) and (b) show the localized allocation and the distributed allocation, respectively. The illustration herein shows a case where the number of sub-carriers for one sub-channel is 12 and six users are frequency-division multiplexed. Of these allocation methods, the localized allocation is suitable for obtaining multi-user diversity gain, whereas the distributed allocation is suitable for obtaining frequency diversity gain.
The transmitting signals that have been allocated onto the sub-carriers (sub-channels) for transmission use at sub-carrier mapping unit 1004 in the terminal apparatus in FIG. 10 are then input to an IDFT (Inverse Discrete Fourier Transform) unit 1005, and transformed from frequency-domain signals to time-domain signals. Then, the signals pass through a P/S (Parallel to Serial) converter 1006 to a CP (Cyclic Prefix) inserter 1007, where CP (the signal generated by duplicating the rear part of the symbol after IDFT) is inserted. Then, the signal is converted into the analog signal at a D/A (Digital to Analog) converter 1008. The resultant is up-converted to a radio frequency band signal at a radio unit 1009 so as to be transmitted from a transmitting antenna unit 1010. The thus generated transmitting signal has the advantage of its PAPR being low compared to a multi-carrier signal.
FIG. 12 shows a base station apparatus configuration for receiving signals transmitted from the terminal apparatus of FIG. 10. As shown in FIG. 12, in the base station apparatus receiving signals of SC-FDMA scheme, the signal received at an antenna unit 2000 is converted to an A/D convertible frequency at radio unit 2001 first. Then, the signal is converted into a digital signal by an A/D converter 2002. Subsequently, a synchronizer 2003 establishes symbol synchronization. Then, after CP is removed from every symbol at a CP remover 2004, the signal passes through a S/P converter 2005, so that the signals in time domain are converted into signals in frequency domain by a DFT unit 2006. The pilot signal for channel estimation (a known signal transmitted together with the data signal from the terminal apparatus), having been converted in the form of a frequency-domain signal is sent to channel estimator 2007, where channel estimation is performed.
The signal the base station apparatus receives is a set of frequency division multiplexed signals transmitted from a plurality of terminals as shown in FIG. 11. A sub-carrier demapping unit 2008, based on the mapping information (the information that specifies the relationships between terminal apparatuses and sub-carriers used by the terminal apparatuses) determined beforehand by a scheduling unit 2012, picks up sub-carriers (sub-carriers that constitute one sub-channel) to be used for every terminal apparatus. Then, in an equalizer 2009, an equalization process for the received sub-carriers collected for each terminal apparatus is carried out based on the estimated channel. Then, after transformation at an IDFT unit 2010 from frequency-domain signals to time-domain signals, the transmitted data for every terminal apparatus is regenerated at a demodulation and error correction decoder 2011.
Also, a pilot signal for reception-level measurement is sent from DFT unit 2006 to scheduling unit 2012. Based on the measurement result on the reception level using this signal, scheduling unit 2012 performs scheduling, taking into account the transmission condition of each terminal. The mapping information determined by scheduling unit 2012 is subjected to modulation and the like at a transmitter 2013, passed through a D/A unit 2014, radio unit 2015 and the like, and then transmitted from an antenna unit 2016 to each terminal. This mapping information is used for transmission of the next frame and afterward on the terminal side.